Although the invention is not merely limited to gamma-cameras, this latter type of device shall be described as an example for disclosing the state of the prior art and its drawbacks. The American patent U.S. Pat. No. 3,011,057 issued to H. O. ANGER describes the general structure and functioning of this type of device.
A gamma-camera generally includes a collimator for limiting the angle of incidence of the gamma photons emitted by the organ, a scintillator crystal for transforming the gamma photons into luminous photons, a set of photomultiplier tubes for transforming the luminous photons into electric pulses, and electronic means to transform these electric pulses into signals with the coordinates X and Y.
A gamma-camera also includes a display unit, generally with a cathodic oscilloscope, controlled by the signals with the coordinates X and Y. Thus unit displays the impact point of the gamma photon on the crystal. It may possibly comprise a photographic device so as to form an image of the organ observed by integrating a large number of luminous points. In addition, it may include a device for digitally processing the images. In particular, the image digital processing device may be used to reconstruct images of sections of the organ examined so as to produce tomographies of these organs. In this latter case, image reconstruction algorithms identical to those used in tomodensitometry are used.
By way of example, the accompanying FIG. 1 shows a gamma-camera, such as the one described in the document FR-A-2 615 961.
The gamma photons (.gamma.) to be detected and localized are emitted by an organ 1. The actual camera includes:
a collimator 2 exposed to gamma radiation, PA1 a film 3 of a scintillator material able to receive the collimated gamma radiation and, by scintillation, produce a luminous radiation 1, PA1 a set of photomultiplier tubes 6 disposed opposed the scintillator, each tube delivering a pulse-shaped electric signal in response to the luminous radiation it has received, PA1 integrators 10 of the pulse signals, these integrators delivering the weighted signals X+, X-, Y+ and Y-, PA1 a good counting percentage response, that is a capacity to process a large number of events per unit of time, PA1 an image quality independent of the energy of the gamma radiation emitted by the isotrope in question.
sets of weighting resistors 7 receiving the electric signals delivered by the photomultiplier tubes 6 and delivering four similar electric signals x+, x-, y+ and y- known as "weighted pulses", the two signals x+ and x- expressing the position of scintillation with respect to a first axis X, and the two signals y+ and y- expressing this position with respect to a second axis y,
circuits 8 for transmitting the similar signals x+, x-, y+ and y- and including, for example, a variable gain amplifier, a delay line and a basic potential restoration signal,
calculation means 20 able to calculate from the weighted signals the coordinates X and Y of a display point corresponding to the scintillation point (s) on the scintillator material; for example, in accordance with the document U.S. Pat. No. 4,672,542, the means 20 calculate the quatities ##EQU1##
a displayer 22 including, for example, a cathodic ray oscilloscope and its horizontal and vertical deviation plates.
The gamma camera shown also includes a circuit 19 connected to the set of weighting resistors and which delivers a signal e reflecting the total energy of the luminous radiation pulse (and no longer the position of this radiation). After passage in an integrator 27, a signal E is obtained reflecting the integrated energy. A threshold comparator 28 delivers a validation signal V used to control the calculation means 20.
it is also possible to extract from the circuit 19 an integration authorization signal I applied to the integrators 10 when the detected energy falls into a required band.
Apart from other factors, a gamma camera shall have:
good spatial resolution, that is a capacity to distinguish small nearby sources,
The spatial resolution depends on the calculation accuracy of the coordinates X and Y. This accuracy in turn depends essentially on physical laws governing the functioning of the various means of the gamma camera. Thus, the interaction of a gamma photon with the scintillator crystal gives rise to a scintillation whose intensity exponentially decreases in accordance with the time involved. The time constant of this decrease is characteristic of the crystal used. For a sodium iodide crystal activated with thallium (NaI, Tl), it is about 300 nanoseconds. This scintillation is simultaneously viewed by several photomultiplier tubes. At a given energy of the incident gamma photon, the number of luminous photons comprising the scintillation obeys the POISSON statistical low. The luminous photons pull up photoelectrons from the photocathodes of the photomultiplier tubes. Also, for a given scintillation, the number of photoelectrons pulled up obeys the POISSON statistical law. The electric contribution of a photomultiplier tube thus follows a POISSON statistical distribution. The mean value of this amplitude depends on the energy of the incident luminous photons.
The determination of the location of the scintillation (representative of the emission site of the gamma photon in the organ observed) is obtained by calculating the center of gravity of the electric contributions delivered by the set of photomultiplier tubes.
For a given scintillation, the most difficult problem to resolve consists of determining as precisely as possible the integral of the center of cavity of the electric contributions over a period of about three times the scintillation decrease time constant. The period of integration depends on the time constant of the crystal. The accuracy of the measurement is tainted with errors due to POISSON statistical fluctuation. In fact, according to the POISSON statistics, the standard deviation of fluctuation of the amplitude of the contributions is inversely proportional to the square root of the number of pulled up photoelectrons. Thus, the longer integration is, that is up to three times the scintillation decrease time constant, the larger is the number of photoelectrons taken into account, the lower is the standard deviation and thus more accurate is any evaluation of the mean value of this contribution.
It shall be observed that the period of integration is directly linked to the quality of the spatial resolution of the gamma camera and that this quality is obtained to the detriment of the counting percentage, that is to the detriment of the number of events taken into account per second.
In many gamma cameras, the integration of signals is embodied similarly, as shown on FIG. 1. When several gamma photons are received during the integration period, they are "stacked", in other words the electric pulses corresponding to them are superimposed, which adversely affects the spatial resolution of the camera. Thus, it is necessary to reject the stacked or superimposed pulses.
A certain number of solutions have been put forward to reject these stacked pulses.
The document U.S. Pat. No. 4,629,895 describes a device comprising, in addition to circuits used to localize scintillations, validation circuits for taking into account scintillations whose amplitude is situated within a predetermined range and which are not followed during the integration period by another scintillation, possibly produced at another location of the crystal, and whose intervention would risk falsifying the localization calculation.
The document U.S. Pat. No. 4,882,680 describes a device for rejecting stacked pulses when the integral of the energy exceeds a certain threshold indicating that a stacking has occured during the integration period.
Other devices use various artifices to subtract any extrapolated or simulated pulse "drags", as described in the documents U.S. Pat. Nos. 4,618,775 and 4,612,443, for example.
It is also possible to use a device for sectioning signals, as described in the document U.S. Pat. No. 4,455,616.
Finally, certain devices use dead time correction circuits, as described in the documents U.S. Pat. Nos. 4,198,986, 4,369,495 and 4,549,866, for example.
However, none of these solutions are satisfactory as they either reject signals and thus degrade the counting percentage performance or subtract any theoretical or extrapolated "drags" or truncate the pulses and then result in degrading spatial resolution.